In general positioning of targets such as wafers on a chuck is known from machining processes, as in EP 0100648, JP7237066 and JP8064662, where wafers are frozen on to a chuck before machining the same. According to abstract of the latter publication, a wafer is mounted to a wafer mounting face through a pure water layer having micro thickness. A disadvantage in respect of the required positioning precision in case of transfer of these known concepts to a lithography system, is the piping required for feeding refrigerant through the chuck.
The preceding disadvantage is illustrated in PCT/US01/26772, which discloses a wafer clamp in a lithography system. The clamp is also used for transport of heat induced by a charged particle beam on a target. The clamping and releasing of the target is in this device performed by applying “one or more” phase transitions to a clamping component that is applied between a wafer and a supporting structure. These phase transitions “facilitate various operations throughout the process” and “ensure that the wafer may be easily loaded and released from the structure. The clamping component is applied in a liquid or gaseous form, and brought into a solid state by active cooling of the support structure, so as to achieve a solid clamping of wafer to said structure. It is here concluded that such a manner of clamping can basically be denoted as gluing the wafer.
The above known clamping device for a lithography system is indicated to be “particularly useful in processes that require wafer cooling”, in particular in vacuum due to a large contact area between component and wafer, and a high thermal conductivity of the clamping component. A disadvantage of this known system however, in addition to the required temperature changes in the support structure, is the plurality of conduits required for separately delivering the clamping component and a circulated chilled fluid to the target table.
US patent publication 2005/0186517 relates to a process for a lithography system, of attaching a wafer to a chuck aligning a wafer to a wafer stage and subsequently exposing the wafer. In particular it teaches to cause opposite stress against wafer expansion after an initial stress relieving expansion of a wafer chuck, thereby potentially doubling the amount of allowed heating of the wafer before undesired slip between wafer and chuck occurs. The attaching process is exemplified by a process using electrostatic clamping and by a process of using vacuum clamping, also requiring cabling and or conduits to a moveable table carrying said target.
Outside the vacuum environment encountered in the field of lithographic exposure of targets, it is from the technical area of wafer testing, known by EP patent application 511928 of 1991 en by presentation publication “Liquid interface at wafer test” presented at the SWTW 2005 conference on Jun. 4, 2005, known to integrate heat conduction and clamping of a wafer to a chuck by means of a flowing film of water. The principle conceived in this known device is to realise conduction of target induced heat through the film of water by maintaining the same at ultimately small thickness, however sufficiently large for levelling out roughness as typically encountered at wafer back-sides. Clamping is in this principle according to the first document realised by fluid transfer on the top side of a grooved, planar clamping section of a chuck. The later document, where the grooves seem to have been omitted form the clamping section, implicitly confirms this explanation by noting that heat is not carried away by fluid flow: “it is conducted through the fluid into the chuck”. Moreover it indicates that a wafer is tightly pulled onto the film by means of a vacuum, which vacuum is applied to a liquid film between chuck and wafer, at recovery locations in the chuck, peripheral to a wafer positioned on the chuck, the fluid thereby entering via a central opening.
It is impractical to carry over this concept known from the field of wafer testing to the field of wafer exposure, due to the vacuum circumstances at which lithographic exposure is performed in modern systems. However also for lithographic systems not performing under severe vacuum conditions, a disadvantage exists in that said flowing of water may be an undesired phenomenon in general, and in particular due to a risk of water leads becoming clogged by contaminations. In case of a required vacuum operation the known system fails due to a lack of over pressure at the top side of the target. Where the recovery openings require some amount of vacuum, or pressure below atmospheric for creating the flow of liquid, the central supply opening for the fluid pre-supposes a pressure higher than that at the recovery side. Should this system be entered into a vacuum environment, the target will tend to be lifted from its position rather than that it will tend to be clamped.